Related Clues:

• Duel helper
• Number of a magazine
• The Phanerozoic, e.g.
• Tiny time
• See 41 Down
• Era after era
• It's split into eras
• See 26 Down
• Quite a long time
• It's longer than an era
• Big time division
• Span of epoch proportions?
• Topic of debate
• Extensive period
• Immeasurable time